This invention relates to electrochemical cells and, more particularly, to a seal for such cells.
The sealing of a cell represents one of the most troublesome and complex manufacturing problems encountered by practitioners of the art. The problem is particularly acute when the internal cell components, such as lithium metal in certain batteries, are extremely chemically active and the seal is hermetic. Besides the theoretical cconsiderations involving the choice of materials, the practical balancing of economics and reliability have plagued the industry. The ideal seal is one which is inexpensive, easily assembled, and resistant to thermal and mechanical shocks.
The extent to which the industry has searched for such a seal may be best demonstrated by the quantity of literature directed to this one portion of the cell.
Conventionally, glass-to-metal seals and ceramic-to-metal seals have been used to hermetically seal electrochemical cells. In the glass-to-metal seal, an aperture is provided in the cover of a cell. A glass bead, having a metallic conductor extending centrally therethrough, is placed in the aperture so that the conductor, called a feedthrough, extends outwardly from both surfaces of the cover. The cover, bead, feedthrough are then fired at a temperature which melts the glass and fuses it at its outer periphery to the cover and internally to the feedthrough. Naturally, the heating of the assemblies to the high temperatures involved represents an expenditure of energy which adds to the expense of the assembly.
Glass-to-metal seals fall into two general types: matched and mismatched. In the matched seal, the glass and metal have the same coefficient of expansion and the glass is bonded to the metal through an oxide layer formed on the surface of the metal. Improper oxide formation adversely effects the integrity of the seal. In the mismatched seal, the glass is retained in the metal body by compressive forces generated by the differences in the coefficients of expansion of the glass and metal.
The ceramic-to-metal seal is generally similar to the glass-to-metal seal except that the ceramic is bonded to the metal by brazing rather than fusing.
While the above seals have been extensively used owing to the quality of the seal, they are expensive to manufacture and easily damaged by thermal and mechanical shocks.
A less expensive seal is disclosed in U.S. Pat. No. 3,109,055 which describes an internally threaded, relatively heavy walled, metallic sleeve which is disposed about an externally threaded, cylindrical, dielectric bushing. The bushing has an axial bore through which a conductor extends. The sleeve is circumferentially deformed along a portion of its length to radially compress the sleeve about the bushing and the bushing about the conductor as the bushing is screwed into the sleeve. The bushing is formed from polytrifluorochloroethylene a dielectric material having practically no molecular water or moisture permeability and which acccordingly is adapted to provide a vapor seal.
An object of the present invention is to provide a hermetic seal which is inexpensive, easy to assemble, resistant to thermal and mechanical shocks, and which occupies a minimal volume so that the volume of electrochemically active cell components can be maximized. As will become apparent, the seal, which we call a "ball and socket" seal need not necessarily be hermetic; by means of a simple material substitution, one may reduce the cost of the seal still further for applications in which hermetic sealing is unnecessary.
Generally, the seal is formed with a cover member having a through-hole. The surface of the cover which surrounds the through-hole forms one portion of a socket, hereinafter described. The remaining portion of the socket is provided by a retention member affixed to the cover. Interjacent the socket-defining surfaces is a member formed from an electrically insulative material which, for hermetic sealing, also possesses a low moisture permeability. The insulating member and retention member both have a through-hole which, together with the through-hole of the cover, accomodate a metallic conductor, or feedthrough. The holes of the cover and retention member are sized so that portions of the insulating member protrude therethrough.
Once the components of the seal have been assembled in the above manner, the socket volume is reduced by compressing the socket walls, forcing the insulating member to generally conform to the inner socket surface, and forming an effective seal.
Further details concerning our seal are discussed in the following Description of the Preferred Embodiment which includes the following Figures.